1. Field of the Invention
The present invention is directed to a method for recovering argon from partial oxidation based ammonia plant purge gases. More particularly, the present invention is directed to a method for recovering argon from a gas mixture comprising argon, carbon monoxide, methane, hydrogen, and nitrogen.
2. Description of the Prior Art
The commercial preparation of argon by separation of air is well known in the art. Equivalent quantities of nitrogen and oxygen are also produced by this method. Because the demand for argon has been growing at a more rapid rate than the demand for nitrogen and oxygen, there is a need for alternative sources for producing argon. One such alternative source is the argon rich purge gas from an ammonia plant.
A conventional method for producing the hydrogen and nitrogen gas mixture for ammonia synthesis is primary steam reforming of natural gas or other hydrocarbon gas followed by secondary reforming of the gas with air. Contaminants in the hydrogen and nitrogen gas mixture, such as carbon monoxide and carbon dioxide, are removed by shift conversion (reaction of carbon monoxide with steam to form additional hydrogen and carbon dioxide), absorption in amines or other alkaline solvents (carbon dioxide removal), and methanation (conversion of trace carbon monoxide and carbon dioxide to methane).
Steam reforming to produce hydrogen consists of treating a hydrocarbon feed mixture with steam in a catalytic steam reactor (reformer) which consists of a number of tubes placed in a furnace at a temperature in the range from about 1250.degree. F. to about 1700.degree. F. The reversible reforming reactions which occur when methane is used as the hydrocarbon feed mixture are set out below. EQU CH.sub.4 +H.sub.2 O.dbd.CO+3H.sub.2 EQU CH.sub.4 +2H.sub.2 O.dbd.CO.sub.2 +4H.sub.2 EQU CO+H.sub.2 O.dbd.CO.sub.2 +H.sub.2
The hydrogen rich gas mixture exiting the steam reformer consists of an equilibrium mixture of hydrogen, steam, carbon monoxide, carbon dioxide, and unreacted methane. The reforming reactions are endothermic and therefore hydrocarbons and process waste gases are burned in the reformer furnace to provide the endothermic heat.
The gas mixture exiting the primary steam reformer is further heated and treated with air in the secondary reformer. Nitrogen in the air is used for ammonia synthesis and oxygen in the air is used to combust unreacted methane from the primary reformer to produce heat. This heat sustains the simultaneously occurring endothermic reforming reactions. The secondary reformer operates at a temperature between about 1850.degree. F. and about 2700.degree. F.
The hydrogen and nitrogen rich gas mixture from the secondary reformer is cooled and treated in a shift converter to aid in the conversion of carbon monoxide to carbon dioxide and additional hydrogen. After being cooled, the shift reactor gases are treated in a solvent absorption system to remove carbon dioxide. After removal of carbon dioxide, the hydrogen and nitrogen gas mixture is treated in a methanator to convert trace carbon oxides to methane. The gas mixture from the methanator is fed to the ammonia synthesis reactor.
Ammonia production processes and hydrogen production processes are disclosed in more detail in "Ammonia and Synthesis Gas: Recent and Energy Saving Processes", Edited by F. J. Brykowski, Chemical Technology Review No. 193, Energy Technology Review No. 68, Published by Noyes Data Corporation, Park Ridge, N.J., 1981, which disclosure is incorporated herein by reference.
Unreacted hydrogen and nitrogen in the ammonia synthesis reactor is recycled to the reactor. During recycle, argon, which enters the reactor via the air added in the secondary reformer, increases in concentration in the reactor gas. A purge stream is therefore periodically released from the ammonia synthesis reactor to remove argon.
Conventional methods for recovering argon from ammonia plant purge gas containing argon admixed with hydrogen, nitrogen, methane, and ammonia have generally focused on cryogenic processes which consist of first pretreating the gas to remove ammonia and then fractionating the gas in three cryogenic distillation columns. The first two columns separate hydrogen and nitrogen from the feed mixture and the third column separates methane to provide a pure liquid argon product and a pure methane product for use as fuel. The first cryogenic distillation column, which separates the bulk of the hydrogen contained in the ammonia purge gas, may be replaced with a pressure swing adsorption or membrane separation system.
U.S. Pat. Nos. 4,687,498, 4,750,925, and 4,752,311, issued to MacLean et al., disclose the recovery of argon from conventional ammonia plant purge gases which contain hydrogen, nitrogen, methane, and argon. The methods comprise removing methane and nitrogen from the purge gas mixture by pressure swing adsorption, then removing hydrogen, and finally cryogenically distilling the mixture to remove remaining amounts of nitrogen and hydrogen thereby preparing a pure argon product.
In a pressure swing adsorption system (PSA), a gaseous mixture is passed at an elevated pressure through a bed of an adsorbent material which selectively adsorbs one or more of the components of the gaseous mixture. Product gas, enriched in the unadsorbed gaseous component(s), is then withdrawn from the bed.
The term "gaseous mixture", as used herein, refers to a gaseous mixture, such as air, primarily comprised of two or more components having different molecular size. The term "enriched gas" refers to a gas comprised of the component(s) of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through the absorbent bed. The enriched gas generally must meet a predetermined purity level, for example, from about 90% to about 99%, in the unadsorbed component(s). The term "lean gas" refers to a gas exiting from the adsorption bed that fails to meet the predetermined purity level set for the enriched gas. When the strongly adsorbed component is the desired product, a cocurrent depressurization step and a cocurrent purge step of the strongly adsorbed component are added to the process.
The term "adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end of a single bed system is the inlet end of the single bed while the inlet end of the two bed system (arranged in series) is the inlet end of the first bed in the system. The outlet end of a single bed system is the outlet end of the single bed and the outlet end of the two bed system (arranged in series) is the outlet end of the second bed in the system. By using two adsorption beds in parallel in a system and by cycling (alternating) between the adsorption beds, product gas can be obtained continuously.
As a gaseous mixture travels through a bed of adsorbent, the adsorbable gaseous components of the mixture enter and fill the pores of the adsorbent. After a period of time, the composition of the gas exiting the bed of adsorbent is essentially the same as the composition entering the bed. This period of time is known as the breakthrough point. At some time prior to this breakthrough point, the adsorbent bed must be regenerated. Regeneration involves stopping the flow of gaseous mixture through the bed and purging the bed of the adsorbed components generally by venting the bed to atmospheric or subatmospheric pressure.
A pressure swing adsorption system generally employs two adsorbent beds operated on cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is in the adsorption or production step, the other bed is in the regeneration step. The two adsorption beds may be connected in series or in parallel. In a serial arrangement, the gas exiting the outlet end of the first bed enters the inlet end of the second bed. In a parallel arrangement, the gaseous mixture enters the inlet end of all beds comprising the system. Generally, a serial arrangement of beds is preferred for obtaining a high purity gas product and a parallel arrangement of beds is preferred for purifying a large quantity of a gaseous mixture in a short time cycle.
Between the adsorption step and the regeneration step, the pressure in the two adsorption beds is generally equalized by connecting the inlet ends of the two beds together and the outlet ends of the two beds together. During the pressure equalization step, the gas within the pores of the adsorption bed which has just completed its adsorption step (under high pressure) flows into the adsorption bed which has just completed its regeneration step (under low pressure) because of the pressure differential which exists between the two beds. The adsorption bed which completed its adsorption step is depressurized and the adsorption bed which completed its regeneration step is repressurized. This pressure equalization step improves the yield of the product gas because the gas within the pores of the bed which has just completed its adsorption step has already been enriched. When more than two beds are employed in the adsorption system, it is common to have a number of pressure equalizations steps.
Gas separation by the pressure swing adsorption method is more fully described in, for example, "Gas Separation by Adsorption Processes", Ralph T. Yang, Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes" Butterworth 1987, and in U.S. Pat. Nos. 2,944,627, 3,801,513, and 3,960,522, which disclosures are incorporated by reference herein. Modifications and improvements in the pressure swing adsorption process and apparatus are described in detail in, for example, U.S. Pat. Nos. 4,415,340 and 4,340,398, which disclosures are incorporated by reference herein.
While the above methods provide processes for producing argon from a steam reformer based ammonia plant, none of the methods disclose the recovery of argon from a purge stream from a partial oxidation based ammonia plant The purge gas from a steam reformer based ammonia plant contains argon admixed with hydrogen, nitrogen, methane, and ammonia while the purge gas from a partial oxidation based ammonia plant contains argon admixed with hydrogen, nitrogen, carbon monoxide, and methane. Argon sources, which contain a high concentration of argon (streams which have an argon concentration higher than 6%), are particularly attractive because of the high value of argon and because such sources provide an opportunity for cost-effective argon recovery. The purge gas from a coal-based partial oxidation plant is one such attractive source of argon.
In a partial oxidation based ammonia plant, coal is gasified with oxygen and steam to yield a crude hydrogen rich gas mixture containing hydrogen admixed with carbon dioxide, carbon monoxide, hydrogen sulfide, argon, and unreacted methane. Argon enters the system with oxygen supplied to the gasifier by a cryogenic air separation plant. Typically, air is separated into an oxygen product and a nitrogen product. Argon distributes between the two products. When argon is not separated in the air separation plant, argon may be recovered from the partial oxidation based ammonia plant purge stream. The crude hydrogen rich gas mixture is treated to recover waste heat, to shift convert carbon monoxide to carbon dioxide, and to remove carbon dioxide and hydrogen sulfide, and other sulfur compounds that may be present The hydrogen rich gas mixture is then cooled to The air separation plant that provides oxygen for the gasification also provides nitrogen for the scrubbing. The liquid nitrogen used in the scrubber also contains argon as an impurity. In the liquid nitrogen scrubber, all of the impurities, such as carbon monoxide, methane, and argon, present in the hydrogen rich gas mixture are washed by the liquid nitrogen. The amount of liquid nitrogen in the scrubber is adjusted so that the vapor stream leaving the top of the scrubber contains stoichiometric amounts of hydrogen and nitrogen required for ammonia synthesis. The liquid product leaving the bottom of the liquid nitrogen scrubber is the waste stream and contains a mixture of argon admixed with hydrogen, nitrogen, carbon monoxide, and methane.
In one process variation, (Reference: Kirk & Othmer, Encyclopedia of Chemical Technology, Vol. 2, page 483), the waste stream from the liquid nitrogen scrubber is subjected to steam reforming and shift conversion and recycled with the hydrogen rich gas mixture from the gasifier. A portion of the waste stream or the recycle stream must be removed as a purge gas to prevent argon from accumulating in the system. This purge gas stream contains argon mixed with hydrogen, nitrogen, methane, and carbon monoxide. In another process variation (Reference: Kirk & Othmer), the waste stream is subjected to further cryogenic separation to separate the stream into various enriched streams for recycle to appropriate locations in the coal-gasification and hydrogen purification plant. Typically, an enriched argon waste stream is removed as a purge gas stream to prevent argon from accumulating in the system. This purge gas stream also contains argon admixed with nitrogen, carbon monoxide, methane, and optionally, hydrogen.
In addition to coal as a fuel source, the partial oxidation ammonia plant may employ alternative fuel sources such as hydrocarbon containing gas, oil, waste products having fuel value, or a mixture of the above fuel sources including coal in the gasification process to generate the hydrogen rich gas mixture.
The present invention provides an improved method for producing argon from partial oxidation based ammonia plant purge gas employing a combination of non-cryogenic and cryogenic separating steps. The present invention also provides a novel pressure swing adsorption method to remove methane, carbon monoxide and most of the nitrogen from argon in the purge gas exiting an ammonia synthesis plant.